The field of the invention relates to the field of electrostatic discharge protection for semiconductor electronic circuits.
Electrostatic protection devices ESDs such as RC-triggered devices that use the conductance properties of a capacitor to switch these protection devices on in response to rapidly changing voltage levels are known. Electrostatic discharge occurs when, for example, a body with a high capacitance touches a circuit and the stored charge in the body is transmitted to the circuit resulting in a rapidly increasing incoming current. This can lead to high voltage changes within the circuit unless the current can be dissipated. If too high a voltage is allowed to manifest itself across an electronic device it may cause breakdown and damage to that device.
Various ESDs have been devised that are responsive to rapidly increasing incoming currents and allow a current to flow through low resistance devices, thereby dissipating the current and reducing any voltage peaks to levels that are not high enough to damage component devices.
FIG. 1a shows an RC-triggered ESD device according to the prior art. This device uses the resistance properties of a capacitor, which are high in a steady state and low when voltages are rapidly changing, to turn the large MOS device on in response to a rapidly changing voltage level and off when the device is in a steady state. Turning the big MOS device on allows a current to flow and a peak voltage level can be dissipated. As the voltage peak relaxes the voltage level stops changing so rapidly and the capacitor stops conducting and will start to charge and the big MOS will turn off and the low resistance path will disappear.
In semiconductor devices it is becoming more common to address different voltage domains without additional mask costs where possible and for this reason input/output circuits often use MOS devices with voltage stress tolerances that are lower than the power supply level, for example 1.8V gate oxide MOS devices might be used with input/output cells supplied at 2.5V.
In such cases care must be taken not to over stress the devices and thereby degrade or permanently damage them. One way of addressing this potential problem is to provide cascaded structures where the devices are arranged in series between the power rails and intermediate power levels are applied to appropriate nodes in order to limit over-voltages. FIG. 1b shows an electrostatic protection device similar to that of FIG. 1a where the devices are formed from 1.8V gate oxide devices and are supplied with an intermediate voltage generated by a voltage divider.